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1. Field of the Invention
The present invention relates generally to servo systems, and more particularly to servo systems actuated by a DC or stepper motor where high speed and highly accurate motion control is required.
2. Description of the Related Art
There are many systems where the physical transport of material requires both a relatively high speed and highly accurate motion control. A specific example used herein to describe the present invention is a servo system such as found in a hard copy apparatus (xe2x80x9cprinterxe2x80x9d hereinafter) print media path (also referred to more simply as a xe2x80x9cpaper pathxe2x80x9d) where translation of a sheet of print medium (xe2x80x9cpaperxe2x80x9d hereinafter) relative to one or more writing instruments (xe2x80x9cpen(s)xe2x80x9d hereinafter) is critical to print quality. The present invention will be described in an exemplary embodiment for an inkjet printer; no limitation on the scope of the invention is intended by the inventors through the use of this exemplary embodiment nor should any be implied therefrom. The art of ink-jet technology is relatively well developed. Commercial products such as computer printers, graphics plotters, copiers, and facsimile machines employ ink-jet technology for producing hard copy. The basics of this technology are disclosed, for example, in various articles in the Hewlett-Packard Journal, Vol. 36, No. 5 (May 1985), Vol. 39, No. 4 (August 1988), Vol. 39, No. 5 (October 1988), Vol. 43, No. 4 (August 1992), Vol. 43, No. 6 (December 1992) and Vol. 45, No. 1 (February 1994) editions. Ink-jet devices are also described by W. J. Lloyd and H. T. Taub in Output Hardcopy [sic] Devices, chapter 13 (Ed. R. C. Durbeckand S. Sherr, Academic Press, San Diego, 1988).
FIGS. 1A and 1B are schematic depictions of an ink-jet hard copy apparatus 101 in accordance with the present invention. A writing instrument 20 is provided with a printhead 14 having drop generators including nozzles for ejecting ink droplets onto an adjacently positioned print medium, e.g., a sheet of paper 22, transported through the apparatus"" printing zone 28. [The word xe2x80x9cpaper is used hereinafter for convenience as a generic term for all print media; the implementation shown is for convenience in explaining the present invention and no limitation on the scope of the invention is intended by the inventors nor should any be implied.] An endless-loop belt 26 is one type of known manner paper transport. A motor (not shown) is used to drive a belt pulley 62. A biased idler 64 provides appropriate tensioning of the belt 26. The belt 26 rides over a platen 42 surface of a vacuum box 40 in the print zone 28. The platen 42, having vacuum ports 44, is associated with a vacuum induction system 48, 50. The paper sheet 22 is picked from an input supply (not shown) and its leading edge is delivered to the vacuum belt 26 (the paper path is represented by arrow xe2x80x9cYxe2x80x9d).
In general, paper motion is usually actuated by a DC-motor or stepper motor coupled to the belt pulley 62. Other paper transport devices such as pinch-drive roller combinations engaging one or more surfaces of the sheet of paper are also known in the art and the present invention can be adapted for use therewith. Note also that while in the state of the art ink-jet printing, where swaths of print are usually scanned sequentially in the x-axis, motion in the y-axis is generally unidirectional. However, it is also known to have bidirectional paper motion in contact pen plotters. It will be recognized by those skilled in the art that he present invention is applicable to both types of technology.
The problem is in producing a relatively rapid motion of the paper in the y-axis in combination with accurate positioning relative to the pens. Design engineers must deal with the various subsystems of the media drive system, namely, gear train (also referred to hereinafter as xe2x80x9ctransmissionxe2x80x9d) wind up, backlash, and lack of viscous damping. Both transmission wind up and backlash will generally result in poor line feed accuracy, contributing to poor image quality. A variety of individual or combinable solutions have been employed in the art.
One prior solution is to provide a direct drive, i.e., a direct coupling between the motor and media. Driving a mechanical system without gear reduction or transmission results in systems that are not optimized. Alternatively, a larger motor must be employed to accomplish what a smaller motor with gear reduction can achieve.
Another solution is to provide mechanisms for inducing friction. Friction intentionally added to a system will inevitably detract from performance because of lack of power, loss of efficiency, and the added expense of a viscous dashpot. Moreover, friction components are difficult to duplicate accurately for mass produced products.
Another solution is to add separate braking mechanisms. Brakes can be actuated only as needed, but require potentially expensive actuators (such as solenoid mechanisms).
Yet another solution is to tune a servo system for over-damped behavior. This is a typical practice in achieving a stable stopping point while avoiding backlash. However, over-damped systems are always suboptimal in speed performance. The load approaches its target position monotonically. This is logical as the load is effectively controlled in one direction; if the load overshoots the target position, the backlash in the gear train will result in over compensation in the opposite direction. This solution does not account for relaxation of transmission wind up.
Still another solution is to use creep motion. Creep moves are specially designed servo motions that are optimized for low speeds. Large portions of servo moves are done under high speed, lightly damped, conditions. As the commanded position is nearing, the servo system enters a creep move behavior in which the system slows to a minimum speed need for a smooth motion (viz, no stick-slip behavior) to the final target position. The slow speed is used to minimize wind-up and distance traveled after the actuation force is moved. This method can be exceptionally slow if the system characteristics (friction, inertia) do not fall within a small tolerance window. Neither does this solution account for relaxation of transmission wind up.
A further solution is the use of anti-backlash gear trains. Anti-backlash gears can eliminate most of the problems produced by the backlash in a gear train, but it does not address the lower proportion of viscous friction with respect to dry friction in many systems. Servos for holding position are not symmetrical since one direction will have a distinctly different compliance. This solution does not address the issue of transmission wind up.
Therefore, there is a need for an active damping and backlash control system for high speed transport systems requiring highly accurate motion control.
In its basic aspects, the present invention provides a servo control system including: a directional load; a primary motor coupled to the load; a secondary motor coupled to the load, wherein during acceleration operation of the load the primary motor and secondary motor drive in tandem, and during deceleration and hold position operations of the load the primary motor and secondary motor drive the load with opposing but not necessarily equal torque.
In another basic aspect, the present invention provides a method for actively damping and substantially eliminating backlash in a servo motion control system, the method including: during acceleration and constant velocity operation on a load, a primary motor and secondary motor drive in tandem, and during deceleration and hold position operations on the load, the primary motor and secondary motor drive the driver with opposing torque.
In another aspect, the present invention provides a hard copy apparatus, having a servo paper transport system, including: at least one printing device; a belt for transporting paper to a position adjacent said printing device; a driver for moving said belt; and a servo system for controlling motion of the driver, the servo system having a primary motor coupled to the driver, and a secondary motor coupled to the driver, wherein during acceleration operations of the driver the primary motor and secondary motor drive in tandem, and during deceleration and hold position operations of the driver the primary motor and secondary motor drive the driver with opposing and not necessarily equal torque.
Another aspect of the present invention is a servo control system including: two motors a primary motor and a secondary motor coupled to a load as a tandem pair, said motors each receiving a different PWM control signal wherein the PWM for the primary motor is equal to the PWM generated by a control fixed-functions plus a predetermined offset and the PWM for the secondary motor is equal to the PWM generated by the control fixed-functions minus the predetermined offset.
Another aspect of the invention is a method for actively damping and substantially eliminating backlash in a servo motion control system, the method including: a primary motor coupled to a load is provided with power that is more than a prescribed control fixed-function, a secondary motor coupled to the load is provided with power that is less than the prescribed control fixed-function wherein the secondary motor is still contributing positively to the acceleration of the load such that during an acceleration portion of a move, desired velocity is greater than the actual velocity of the system via providing a large positive PWM to create desired acceleration to overcome inertia and friction.
Another aspect of the present invention provides a hard copy apparatus, having a servo paper transport system, including: at least one printing device; a belt for transporting paper to a position adjacent said printing device; a driver for moving said belt; and a servo system for controlling motion of the driver, the servo system having a primary motor coupled to the driver, and a secondary motor coupled to the driver, wherein said two motors are coupled to the driver as a tandem pair, said motors each receiving a different PWM control signal wherein the PWM for the primary motor is equal to the PWM generated by a control fixed-functions plus a predetermined offset and the PWM for the secondary motor is equal to the PWM generated by the control fixed-functions minus the predetermined offset.
Some of the advantages of the present invention are:
servo control systems in accordance with the invention can be driven at higher speeds than prior art without sacrificing stopping and holding accuracy;
higher accelerations are achieved by combining the power of two motors;
the rate of deceleration is increased by using a secondary motor as an active brake;
decelerations are no longer dependent on system friction which can vary from unit to unit and over time and operating conditions;
servo designs in accordance with the present invention substantially eliminate the need to reduce performance of a product to meet design margins, eliminating the worst case scenario design methodology;
stopping accuracy is improved because of the ability to influence the load in two directions equally during deceleration and hold functions;
holding accuracy is improved as disturbances, such as vibrations, will not cause the gear train to unload with resultant load creep, i.e., the load is held actively in place with no chance of unloading the gear train;
continuous drive loading substantially eliminates backlash, overshoot, and ringing all of which result in poor stopping accuracy;
The foregoing summary and list of advantages is not intended by the inventors to be an inclusive list of all the aspects, objects, advantages, or features of the present invention nor should any limitation on the scope of the invention be implied therefrom. This Summary is provided in accordance with the mandate of 37 C.F.R. 1.73 and M.P.E.P. 608.01(d) merely to apprise the public, and more especially those interested in the particular art to which the invention relates, of the basic nature of the invention in order to be of assistance in aiding ready understanding of the patent in future searches. Other aspects, objects, advantages, and features of the present invention will become apparent upon consideration of the following explanation and the accompanying drawings, in which like reference designations represent like features throughout the drawings.